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Karyotyping
Overview A karyotype is a way of viewing the complete set of an organism's chromosomes under a light microscope. It displays the number as well as physical appearance of the chromosomes which is useful in determining large structural or numerical abnormalities. Karyotyping was first completed in 1955 as part of Albert Levan's lab in determining that the correct number of chromosomes in the human karyotype was 46 not 48 (1). Today karyotyping is still widely used as a determinate method for discovering chromosomal abnormalities and disorders within a fetus for a couple experiencing frequent miscarriages or history of genetic complications. Preparation 1. Cell Collection In order to create a human karyotype cells of interest must first be obtained from the person of interest. Mononuclear cells can be obtained from a number of different sources including bone marrow, blood, amniotic fluid, or the placenta of a developing fetus. Cells can be collected via amniocentesis, a bone amrrow biopsy, chorionic villus sampling, or by a venipuncture depending on the source (2). 2. Cell Processing The processes for cell prepartion varies slightly for different cell types but generally follows a similar protocol. For example, once lymphocytes are obtained from a blood sample, heparin is added to prevent blood coagulation. Then the cells are purified by centrifugation in order to obtain the cells of interest (WBCs) and eliminate other cells found in blood such as red blood cells and granulocytes. After this stage the cells are then placed on a cell growth medium with the addition of phytohemaglutinin (which allows the cells to divide rapidly) for about 3-4 days to obtain the maxium amount of dividing cells. At this point a hypotonic solution is added to the cells to cause them to swell to allow better viewing as well as a colcemid drug which causes the cells to arrest mitosis in metaphase (3). This is optimal for viewing chromosomal pairs under the microscope. 3. Fixation and Staining At this point the cells are allowed to fix (dry) onto a microscope slide to prevent them from moving in solution during viewing. After fixed a stain is used, such as Giemsa to allow individual chromosomal banding patterns to show in order to recognize each individual chromosome pattern/pair partner. When placed under the microscope, chromosomes which are in good condition are placed with their corresponding "partner" chromosome. In human karyotypes the autosomal chromosomes are paired in numerical order from 1-22 followed by the 2 sex chromosomes. This allows one to easily identify abnormal differences in banding patterns as well as aneuplodies. Usages The main purpose of karyotyping today is to determine large chromosomal abnormalities such as aneuploidies (incorrect number of chromosomes) or defect in chromosome structure (large viewable translocations, insertions , deletions of a section of a chromosome). Most of these defects are not viable for live in humans, or result in extreme phenotypic abnormalities. Therefore it is mainly used in order to determine these problems in a developing fetus if a chromosomal defect is suspected, if frequent miscarriages occur, or if the expecting mother is 35 years of age or older. References 1. "Karyotype." Wikipedia. Wikimedia Foundation, 12 Aug. 2014. 2. Haldeman-Englert, Chad. "Karyotyping." MedlinePlus. U.S. National Library of Medicine, 2 Nov. 2012. 3. Bowen, R. "Preparing a Karyotype." Preparing a Karyotype. Colorado State University, 17 Aug. 2000. 4. "The Normal Human Karyotype." Oxford Journals. The Journal of Heredity, Aug. 1963.